"else if" doesn't work

I have made this program which generally works fine unless I enter a string with a length of over 50 or a 0 string. What is going on with my else if statements? I can't find the problem although I checked it many times. Here's the code:

    #include<stdio.h>
main()
{
        char string1[1000] ,i, k,j, sum;
        int length;
        printf("Dwse leksh\n");
        scanf("%s", string1);
        sum=0;
        for(i=0; string1[i]!= '\0'; ++i)
        {
        length=i+1;
        }
         printf("Length of string is %d\n", length);
        if(length>0 && length<=50)
        {
                k=length-1;
                for(j=0; j<k; j++)
                {
                        if(string1[j]==string1[k])
sum=sum+1;
                        k--;
                }
                if(sum==length/2 || sum==length-1)
                {
                        printf("The word is palindrome\n");
                }
                else
                {
                        printf("The word is not palindrome\n");
  }
        }
        else if(length=0)
        {
                printf("Error: mideniki sumboloseira");
        }
        else if(length>50)
        {
                printf("Error: uperbash tou oriou 50 xaraktirwn");
        }
}